4.2: Quorum Sensing

Community behaviors initiated at the unicellular level often involves a process known as quorum sensing. Through this process organisms estimate the density (number of individuals per volume) in their immediate environment. Each individual secretes specific molecules; they also respond to that molecule through specific receptors. The organisms' response is dependent on the signaling molecule's extracellular concentration. More importantly, the response (blue line →) is non-linear; it displays a "threshold" effect (red line → linear response - X axis signal concentration, Y-axis response). Below the system's "threshold" concentration there is little if any cellular response; above the threshold concentration the cells respond fully. When cells or organisms are present at a low density, the signal molecule's concentration is below the threshold concentration. As the density of organisms increases, the molecule's concentration increases and can exceed the threshold concentration; that is when interesting things begin to happen. There can be changes in cellular behaviors, generally associated with changes in gene activity.^[181] We can think of non-linear (threshold) responses as a strategy to avoid over-reacting to minor environmental fluctuations or when there are not enough organisms around to produce a useful cooperative response. Only when the signal gets high enough does the system respond. The threshold concentration reflects the signaling molecule' binding affinity to its target receptor, and other factors influencing molecular stability and diffusion.

A classic example of threshold behaviors is provided by the light emitting marine bacteria Vibrio fischeri. These bacteria colonize a dedicated organ of the Hawai'ian bobtail squid (Euprymna scolopes) shortly after the squid "hatch".^[182] The colonization process has a number of steps. The bacteria enter and adhere to an opening leading the light emitting organ. The bacteria begin to divide; as their numbers increase they pass a threshold and begin to secrete molecules that form of gooey matrix, known as a biofilm. The bacteria swim through the biofilm toward signals released by the light organ cells (a process known as chemotaxis). The result is that bacteria enter and colonize the light organs.

Within the light organs the bacteria emit light through a reaction system involving the molecules luciferin and O₂ (→). The light emitting reaction is catalyzed (that is, sped up) by the enzyme luciferase, a protein catalyst. The luciferase protein is encoded by a bacterial gene. Its original role has been proposed to be in the “detoxification of deleterious oxygen derivatives".^[183]

A small number of bacteria would emit little light, not enough to be useful, while using energy (all costs, no benefit). The components of the light emitting system are regulated so that they are synthesized only when the numbers of bacteria are high enough to make the emission of light useful, which decreases the cost to benefit ratio. As with Dictyostelium, the bacteria use a quorum sensing system. Each bacterium releases a signaling molecule; as the numbers of bacteria increase, the signaling molecule's concentration increases and passes through the response threshold. Receptors on the bacteria are activated leading to the synthesis of the components of the light reaction, luciferase and the proteins involved in the synthesis of luciferin. In the presence of O₂ and ATP (in the bacteria) light is emitted. Nutrients to feed the bacteria are supplied by the squid.

Mechanistically similar systems are involved in a range of processes including the generation of toxins (virulence factors), secreted digestive enzymes, and antibiotics directed against other types of organisms. These are produced when the density of bacteria rises above a threshold concentration. This ensures that the synthesis of biologically costly molecules, such as luciferase and luciferin, occurs only when they are likely to be useful and effective – that is, they are produced at a level high enough to carry out their intended roles. Such high concentrations can only be attained through cooperative behaviors involving many individuals.

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